The present invention relates to subterranean stimulation operations and, more particularly, to apparatuses and methods for improving the reliability of pinpoint stimulation operations.
To produce hydrocarbons (e.g., oil, gas, etc.) from a subterranean formation, well bores may be drilled that penetrate hydrocarbon-containing portions of the subterranean formation. The portion of the subterranean formation from which hydrocarbons may be produced is commonly referred to as a “production zone.” In some instances, a subterranean formation penetrated by the well bore may have multiple production zones at various locations along the well bore.
Generally, after a well bore has been drilled to a desired depth, completion operations are performed. Such completion operations may include inserting a liner or casing into the well bore and, at times, cementing a casing or liner into place. Once the well bore is completed as desired (lined, cased, open hole, or any other known completion) a stimulation operation may be performed to enhance hydrocarbon production into the well bore. Where methods of the present invention reference “stimulation,” that term refers to any stimulation technique known in the art for increasing production of desirable fluids from a subterranean formation adjacent to a portion of a well bore. Examples of some common stimulation operations involve hydraulic fracturing, acidizing, fracture acidizing, and hydrajetting. Stimulation operations are intended to increase the flow of hydrocarbons from the subterranean formation surrounding the well bore into the well bore itself so that the hydrocarbons may then be produced up to the wellhead.
One suitable hydrajet stimulation method, introduced by Halliburton Energy Services, Inc., is known as the SURGIFRAC and is described in U.S. Pat. No. 5,765,642. The SURGIFRAC process may be particularly well suited for use along highly deviated portions of a well bore, where casing the well bore may be difficult and/or expensive. The SURGIFRAC hydrajetting technique makes possible the generation of one or more independent, single plane hydraulic fractures. Furthermore, even when highly deviated or horizontal wells are cased, hydrajetting the perforations and fractures in such wells generally results in a more effective fracturing method than using traditional perforation and fracturing techniques.
Another suitable hydrajet stimulation method, introduced by Halliburton Energy Services, Inc., is known as the COBRAMAX-H and is described in U.S. Pat. No. 7,225,869, which is incorporated herein by reference in its entirety. The COBRAMAX-H process may be particularly well suited for use along highly deviated portions of a well bore. The COBRAMAX-H technique makes possible the generation of one or more independent hydraulic fractures without the necessity of using mechanical tools to achieve zone isolation, can be used to perforate and fracture in a single down hole trip, and may eliminate the need to set mechanical plugs through the use of a proppant slug or wellbore fill.
Current pinpoint stimulation techniques suffer from a number of disadvantages. For instance, during hydrajetting operations, the movements of the hydrajetting tool generally reduces the tool performance. The movements of the hydrajetting tool may be caused by the elongation or shrinkage of the pipe or the tremendous turbulence around the tool. The reduction in tool performance is generally compensated by longer jetting times so that a hole is eventually created. However, the increase in jetting times leads to an inefficient and time consuming hydrajetting process.
The COBRAMAX-H process also suffers from some drawbacks. Specifically, the COBRAMAX-H process involves isolating the hydrajet stimulated zones from subsequent well operations. The primary sealing of the previous regions in the COBRAMAX-H process is achieved by placing sand plugs in the zones to be isolated. The placement of sand plugs, particularly in horizontal well bores, requires a very low flow rate which is difficult to achieve when using surface pumping equipment designed for high rate pumping operations. Moreover, when the operating pressures are high, the orifices of the tool must be very small to create a low flow rate. The small size of the orifices makes them susceptible to plugging.
Additionally, the placement of sand plugs in horizontal or substantially horizontal well bores may be difficult. Specifically, current methods of placement of sand dunes in horizontal well bores entail slowly pumping the sand down the well bore as shown in FIG. 1. An artificially low flow rate 2 is used to allow dropping of sand to the bottom side of the casing 4 to form a sand dune 6. To that end, the terminal velocity of the sand dropping down has to be faster than the flow velocity reaching the fracture point. However, this approach may prove ineffective. As shown in FIG. 1, as a sand dune 6 is created, the area above the dune becomes smaller, thereby increasing the flow velocity over the sand dune 6. The increased flow velocity destroys the top of the sand dune 6. As a result, the flow that passes on top of the sand dune 6 may enter the fracture 8 and further open it.
Finally, the SURGIFRAC process which uses the Bernoulli principle to achieve sealing between fractures poses certain challenges. During the SURGIFRAC process, the primary flow goes to the fracture while the secondary, leakoff flow, is supplied by the annulus. In some instances, such as in long horizontal well bores, a large number of fractures may be desired. The formation of each fracture results in some additional leakoff (i.e., seepage). Consequently, with the increase in the number of fractures, the amount of the secondary leakoff flow increases and eventually can significantly reduce the amount of the primary flow to the new fracture. The increased fluid losses reduce the efficiency of the operations and increases the cost. Accordingly, a flow limiter device is desirable to reduce annulus flow requirements while maintaining pore-pressure and limited flow influx to previous fractures below the new fracture, and after pumping has ceased, to let the new fracture slowly close without producing proppant.